Heavy fighting is continuing in Abidjan where forces loyal to Cote d'Ivoire's presidential rivals are battling for control of the country's main city.

Fighters supporting Alassane Ouattara, internationally recognised as Cote d'Ivoire's legitimate president, launched an assault around the presidential palace early on Tuesday as they attempt to oust incumbent president Laurent Gbagbo from power.

Machine gun and heavy weapons fire could be heard near the palace before dawn, witnesses told Reuters, while a spokesman for Ouattara's government has claimed its troops have already occupied Gbagbo's official presidential residence.

"Yes, they are inside his residence. They are in control," Patrick Achi told Reuters by telephone. "But if Gbagbo's there or not, I do not know."

The clashes are the fiercest since fighters supporting Ouattara entered Abidjan five days ago, having already claimed control of many of the country's other major towns.

Meanwhile, United Nations and French military forces also launched attacks against Gbagbo's heavy weaponry on Monday claiming the weapons had been used to target civilians.

Explosions rang out near the presidential palace and at three strategic military garrisons, including the Akouedo military camp.

"On Monday we started an operation to neutralise heavy weapons used to attack the civilian population and UN peacekeepers in Abidjan," Hamadoun Toure, a UN spokesman, told Al Jazeera.

"Heavy weapons used by forces loyal to Gbagbo are stored in various camps in Abidjan. These sites are the targets."

He said the UN mandate is to protect civilians and that its forces would not intervene in fighting between Gbagbo troops and forces loyal to Ouattara.

"If it's fighting between the two armies we don't intervene, because we're impartial, we're neutral," he said.

French President Nicolas Sarkozy said in a statement that he had authorised the 1,600-strong French force based in the country to help in the operation following an appeal from UN Secretary-General Ban Ki-moon, who said the use of force was necessary to prevent further attacks on civilians.

"In the past few days, forces loyal to Mr Gbagbo have intensified and escalated their use of heavy weapons such as mortars, rocket-propelled grenades and heavy machine guns against the civilian population in Abidjan," Ban said in a statement.

Following four months of attempts to negotiate Gbagbo's departure, the UN Security Council unanimously passed a resolution giving the 12,000-strong peacekeeping operation the right "to use all necessary means to carry out its mandate to protect civilians under imminent threat of physical violence ... including to prevent the use of heavy weapons against the civilian population."

Al Jazeera's Haru Mutasa, reporting from Bassam, just outside Abidjan, said there were suggestions that the UN and France were supporting Ouattara, making it easier for his forces to gain ground in Abidjan.

"There are reports that Nicolas Sarkozy, the president of France, had a phone call with Ouattara earlier today," she said. "We don't know what was said but this is an indication that there is some kind of relationship between France and Ouattara."

Ouattara's forces have effectively cornered Gbagbo and his closest supporters after four days of fierce fighting.

Al Jazeera's Mutasa said the people living outside Abidjan were "on edge" after hearing that an "all-out assault by Ouattara's forces is imminent".

"The disturbing thing is the checkpoints and barricades being manned by young men who are unemployed, some of them intoxicated, armed with machetes, [who] decide who goes in and out of Abidjan," she said.

Massacre investigated

UN humanitarian chief Valerie Amos said on Monday that investigators from the world body were probing allegations that hundreds of people have been massacred in the town of Duekoue.

The International Committee of the Red Cross said on Saturday that at least 800 people were killed in intercommunal violence there last week.

The UN peacekeeping mission said the same day that pro-Ouattara forces had killed 220 people and pro-Gbagbo militia fighters killed more than 110 people in Duekoue.

Amos, who is visiting the town as part of a trip to establish humanitarian needs in the country, told Reuters she could not confirm that as many as 800 people had been killed.

"What we do have from the investigations that are being conducted by our colleagues on the human rights side of the United Nations is that they found a mass grave," she said.

"They already found nearly 200 bodies in that grave, and they have found bodies in other parts of the town as well."

Duekoue, which lies in the cocoa-growing belt of western Cote d'Ivoire, was captured by Ouattara's forces on March 29.

Amos said she could not say who was responsible for the killings. People she had spoken to had variously blamed them on Ouattara's forces, fleeing pro-Gbagbo forces and local militia, as well as conflict between natives and non-natives.